The Thermochemical Conversion of Biomass into High-Value Products: Microwave Pyrolysis - The Economic Utilisation of Food Co-Products - page 59

Table 3.7 Bio-oil upgrading routes.

Upgrading Target Needed Effects

Esterification Acids Catalyst þ alcohol Reduced corrosiveness

Improved stability
Increased energy content

d n 9 r 3 n g | 8

Hydrogenation Alkenes

aromatics

Catalyst þ hydrogen Increased energy content

Improved stability

Reduction Aldehydes Catalyst þ hydrogen Improved stability

Potential for using alcohol
products to esterify acids

3.5.4 Pyrolysis Oil Upgrading
The negative properties of bio-oil such as high viscosity, poor storage stability
and corrosiveness stand in the way of the large-scale uptake of bio-oil as a
conventional oil substitute. As a result, much research has been carried out into
processes that will make it more suitable for the desired application. The main
upgrading routes listed in the literature include hydrogenation, esterification,
hydrodeoxygenation, catalytic cracking of the pyrolysis vapours, steam
reforming and the physical emulsification of bio-oil with diesel. Although
hydrogenation and esterification are less atom-ecient processes they are much
simpler than the others, all of which require complicated expensive procedures
and rely on the use of catalysts

3 .

that are subject to fouling during

processing. 79-81
General upgrading schemes for pyrolysis oils include (also see Table 3.7):

esterification of acids to reduce acidity increase stability and to add alkyl
groups in order to increase energy content (residual alcohols can also
improve homogeneity and flow characteristics);
hydrogenation of unsaturated groups to provide additional hydrogen and
therefore energy; 82
reduction of aldehydes to improve stability and burning characteristics.

In addition, the simple addition of small alcohols has been shown to improve
stability and fuel performance. 55

3.6 Summary
Microwave heating offers a uniform, rapid, instantaneous, controllable and
tuneable method for heating materials and thereby initiating chemical
conversions. From the wide range of examples the almost universal appeal of
this method can be observed. The major flaws of microwave heating have been
turned to advantages. Limited penetration means small continuous reactors
can be employed, thereby minimising the inventory of active material means a
safer system. The relatively high energy cost is offset by cleaner products in
higher yields and in some cases products that could not otherwise be observed.